Wave translation system



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WAVE TRANSLATION' SYSTEM Filed Jan.. 25, 1934' 'ATTORNY Taf-MRE@ BV AAAA March 10, 1936;

Patented Mar. l0, 1936 13 claims.

This invention relates to wave translation, and I 1.,-, vention may be an amplier of the general type,

disclosed for example, in the copending application of H. S. Black, Serial No. 606,871, filed April 22, 1932, for Wave translation systems, the paper on Stabilized Feed-Back Ampliers by H. S. Black in Electrical Engineering, January, 1934, pages 114 to 120, H. Nyquist Patent 1,915,440, June 27, 1933, British Patent 317,005, or British Patent 371,88*!in which waves including waves of the frequency range of the fundamental orapplied waves, are so fed from the output to the input of the amplifier as to reduce their output amplitude and render modulation or distortion generated in the ampliiier less than without feedback. It has been fc'und that, by this negative 3o feedback and the above mentioned operation of the amplier as a class B amplifier or operation with the interruptions of plate current, an important improvement can be obtained in emciency, without entailing increase of harmonics or other distortion products. l

The invention, in its aspect referred to above, ,may be a` push-pull amplier if desired, i. e., a duplex amplier circuit withthe input voltage applied to the balanced input circuits in phase v opposition and the energy from the balancedl output circuits combined cumlatively for odd order voltage components and diiferentially for even order voltage components. It has been found .that the push-pull or balanced operation can increase the permissible negative grid bias and yield important increases in eiciency and in distortion' reductionl If desired, the waves tobe fed back from the amplifier output to the amplifier input may be derived from the icombined energy of the balancedoutput circuits, so that the balancing action reduces the feedback of even order products o f modulation. 'I'he balancing before feedbackhas been found eiective in reducing modulation. Such balancing prevents passage of direct current around the feedback loop,

wavaraANsLArroN SYSTEM Enoch B. Ferrell, akhurst, N. I., or to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 25, 1934, Serial No. 708,211

Other objects and aspects of the invention will be apparent from the following description and claims.

`Fig. 1 shows an example ofl a push-pull, negative 'feedback amplifier embodying the `aspect of 5 the invention referred to above; and

. Figs. 2 and 3 show modifications of the amplifier of Fig. 1.

The ampliier of Fig. 1 is shown by way of example as a three-stage amplifier with screen grid 10 tubes I and l in the rst stage and 2 and 2' in the lsecond stage. Tubes 3 and v3 in the third stage are operated as a class B amplifier stage (i. e. operated with their steady grid potential just suicient to extinguish or to 'nearly extinguish 15 their plate current in the absence of input from the preceding stage) or are operated with their grids driven by the'excitation voltage to voltages beyond the cut-01T voltage, i. e..the voltage just suicient to cut off the plate current. Thus, the 20 plate current of each of the tubes l3 and 3' ows in pulses and becomes zero at times during operation of the amplifier, the duration of the periods for which the space currentv of the tube is zero. depending upon the duration of `the 25 swings of the exciting voltage below the value which, when added to the value of any Asteady or biasing voltage employed, gives a'grid voltage equal to the cut-oil? voltage. The amplier of Fig.` 1 is suitable for various purposes. For ex- 30 ample, it maybe employed as a speechI amplier, or amplifier of audio-frequency waves of a fre-- quency band including the 4speech frequency range, the last stage serving as a power amplier. The incoming circuit 4 then may be, for 35 instance, a source of waves of speech frequencies, connected to input transformer 5;v and outgoing circuit G connected to output transformer 1 may be any suitable load circuit for utilizing the amplier wave of the speech or Aaudio frequency 40 band. As a further. example, the amplifier of n Fig. 1 may be employed, if desired, as an amplifier for carrier waves of one or more channels,

and the incoming circuit 6 then may be, for instance, a source of carrier waves of one or more 45' speech channelswith or without associated carrier frequencies, the outgoing circuit 6 being any suitable load or work circuit. The coupling between the stages of the amplifier is shown as of control of the phase of the feedback voltage and therefore is often desirable" provided the class B or last stage does not draw 'appreciable grid current. In Fig. 1, waves are fed from the output the resistance-capacity type, which facilitates 50` of the amplier to its input through a path extending from across the primary winding of the output transformer 'l through conductors 8 and 8. 'resistances 9 and 9' and` stopping condensers I0 and I0' to input bridge networks Il and Il'. Adjustable resistors I2 and I2' across this feedback path control the amount of feedback. 'Ihe input bridges render this feedback path and the incoming circuit 4 conjugate to each other, preventing the feedback from affecting the input impedance of the amplifier. as pointed out in the above mentioned copending application or the above mentioned British Patent 371,887, or the above mentioned papfr by H. S. Black. Plate and screen potentials for the tubes of the amplii'ler may be supplied, for example, from potentiometers shown connected across a voltage source I5, which may be a direct current generator, a rectifier or other suitable voltage source. The stopping condensers I0 and I0 not only prevents the grids of tubes 3 and 3' from being affected by the steady potentials that the plates of those tubes assume in the absence of waves incoming from circuit 4, but moreover prevent passage of the direct current component of the output wave of the amplifier around the feedback loop. The push-pull or balancing action of the ampliiler also prevents this direct current component from passing around the feedback loop. 'Ihe loop gain of the amplifier or gain in a single trip around the loop may be large compared to unity, for producing large reduction of distortion by the negative feedback; and the gain of the forwardly transmitting portion of the amplier, i. e., the gain of the amplifier from the grids of the first stage to the fictitious .plate generator of the space paths of the last stage may be large compared to unity, for providing large overall amplifler gain nothwithstanding the large reduction of distortion and gain by the large amount of negative feedback.

The class B stage or last stage of the amplier may be any one of several types. For example, in one type of tube a comparatively high p is used and the tube is operated near cut-oil with zero grid bias, grid biasing battery 20 and its bypass condenser 2| shown in Fig. 1 being omitted. In a second type a tube of .low a is used and a large bias provided, for example, by battery 20 by-passed by condenser 2|, is required to bring it to cut-off. The first type has the advantage of requiring no extra equipment to supply bias. This is important in low power sets where the other tubes of the amplifier can be biased by means of resistors such as Il, I1 and II between cathode and ground and thus all bias supply equipment avoided. However, with this first type the grid goes positive for half the cycle and thus throws a heavy and distorting load on the previous stage. In high power installations this load may become quite serious because of the size of driver stage required. It is then desirable to use in the last stage a low u tube of such characteristics that its full rated power output can be obtained without driving the grid positive. In high power sets, the saving effected by reducing the required power output of the driving stage will usually more thanv od'set the added expense of the bias supply. Compromise can be, made between these two types of tube for use in the last stage, but then the tube requires a biasing voltage as well as a large amount of driving power to drive its vgrid positive.

If the tube is operated near cut-off, plate currenil-l will flow for a little more than half of each cyc e.

To facilitate obtaining a good compromise between low distortion and low stand-by power consumption, it is often desirable to choose a value of grid bias that allows some idle current to flow,

rather than to choose a value of grid bias that extinguishes the plate current in the absence of grid excitation voltage.

Figs. 2 and 3 show modifications of the circuit of Fig. 1 as regards the manner of obtaining the voltage to be fed back. f Fig. 1 shows shunt feed- `back from the primary winding of the output transformer 1; Fig. 2 shows shunt feedback from the secondary winding of the transformer; and Fig. 3 shows series feedback from the load on the secondary winding of the transformer, the feedback voltage being proportional to the load current. 'Ihe broken line X-X in Figs. 1, 2 and 3 indicates that the systems of these three gures are the same except as regards the portions shown to the right of that line. However, the stopping condensers such as l0 and I0 may be omitted when the voltage to be fed back is obtained as in Fig. 2- or Fig. 3, since the output transformer 'l does not transmit direct current from its primary winding to its secondary winding. l

In each of the three figures, the voltage to be fed back is obtained :from the combined outputs of the two sides of the push-pull or balanced system, so that the balancing action reduces the feedback of even order products of modulation,

and prevents the direct current component of the output from passing around the feedback loop.

What is claimed is:

1. A wave translating system comprising an electric space discharge device having anode, cathode and discharge control means, means for supplying anode potential for said device, means for normally biasing said control means to a. potential of algebraic value at least as high as the critical value at which anode current extinction takes place, a source of input waves for said device of amplitude such that anodecurrent of said device is extinguished during an appreciable portion of their negative swing, and means for producing negative feedback in said device and normally rendering its loops gain large compared to unity for fundamental wave components.

2. A'wave translating system comprising an electric space discharge device, means for producing space current in said device, means normally providing for said device space current reducing effect substantially Just suillcient to reduce space current of the device to zero value, and means for producing negative feedback. in said device. l

3. A wave translating system comprising an electric space discharge device having anode, cathode and discharge control means, means for producing anode current in said device, means for normally biasing said control means to a potential substantially just sufficient to reduce anode current of said device to zero value. and means for producing feedback in said device of waves that reduce the magnitude of its output waves below their value without feedback.

4. A wave translating system comprising an electric space discharge device having anode, cathode and grid structure, means for producing negative feedback in said device and normally rendering its loop gain large compared to unity for vfundamental wave components, and means for biasing said grid structure to a potential substantially Just sufficient to reduce space current oi' said dev ice to zero value.

5. A wave translating system comprising an electric space discharge device, a source of input waves therefor, means for causing space current of said device to be zero during an appreciable portion of the negative swing of the waves from said source but finite in the absence of said waves, and means for producing feedback in said device of waves that reduce'the magniture of its output -ivaves below their value without feedback.

6. A wave translating system comprising an electric space discharge device having anode, cathode and grid structure, a source'of input waves for said device, means for producing negative feedback in said device, and means for normally biasing said grid structure to a potential that causes anode current of said device tov be finite in the absence of said waves but to, be

extinguished during a considerable portion of the negative swing of the waves from said source.

7. A wave translating system comprising an electric space discharge device having anode, cathode and discharge control means, means for supplying anode potential for said devicemeans for normally biasing said control means to a negative potential of numerical value at least as low as that at which anode current extinction takes place, a source of input Waves for said device of amplitude such that anode current of said device is extinguished during a considerable portion of their negative swing, and means for producing negative feedback in said device of waves to the exclusion of direct current.

8. A wave translating system comprising an electric space discharge device having anode,

cathode and grid structure, means for supplying anode potential for said device, means for normally biasing said grid structure to a negative potential of numerical value at least as low as that at which anode current extinction takes place, a source of input waves for said device of amplitude such that anode current of said device is extinguished during a considerable portion of their negative swing, means cooperating with said device to form a closed feedback loop for producing negativey feedback in said system,

and means in said loop for preventing passage of direct-current around the loop.

-9. A wave translatingi system comprising a wave source, a load, electric space discharge devices in push-pull connection, with their input.

and their output connected respectively with said source and said load for transmission through the devices from the source to the load, means for interrupting space current of each of said devices during a considerable portion of the negative swing of the waves from said lsource 4with respect to the device, and means for profducing in said devices negative feedback of waves I 3 that reduce distortion in saidtransmission. and

that are derived from the'combined push-pull output of said system, whereby even-order cornponents of the output Waves of said devices are reduced by the balancing action of the push-pull connection before being fed back.

l0. A wave translating system comprising a wave source, a load, electric space discharge devices in push-pull connection, with their input and their output connected respectively with said source and said load for transmission through the devices from the source to the load, means for interrupting space current of each of said devicesduring a considerable portion of the negative swing of the waves from said source with respect to the device, and means for producing negative feedback in said devices of Waves that reduce distortion in said transmission caused by space current interruptions. I

11. A wave translating system comprising a Wave source, a load, electric space discharge devices in push-pull connection, with their input `and their output connected respectively with said source and said load for transmission through the devices from the source to the load, means for causing space current of each of said devices to be zero during a considerable portion of the negative swing of the waves from said source with respect to the device but iinite in the absence of said waves, and means for producing negativefeedback Iin said devices of waves that reduce distortion in said transmission.

12. In an amplifier operated in accordance with so-called class B operation, in which such control-electrode potential is used that the space current is finite in the absence of waves to be amplified but is interrupted for the order of half the period of the waves being amplied, means for improving the operation comprising means feeding back a portion of the output waves in gain-reducing phase and in sumcient amount to reduce the distortion below the distortion level without feed-back.

13. In combination, an amplifier, comprising space discharge devices in push-pull relation,

operated in accordance with so-called class B operation in which such control-electrode potential is used for each of the devices that the space current of the device is interrupted during a considerable portion of the negative swing of the waves to be amplified, and means for improving operation of the amplifier comprising connections for feeding back a portionA of the output waves in gain-reducing phase and in amount sufficient to reduce the distortion below the 'distortion level without feed-back.

ENOCH B. IEERRELL. 

